"It is neither cold nor hot in fall."
Actually, the pinyin is "bu leng bu re". You are expecting "不冷不熱". Even though your proposed translation is literally "not cold not hot", this instead transforms the given sentence to "It (The temperature) is alright (in the middle)". The more casual translation would then be "It is nice!" if you do not include 也 in the translation.
For the Duolingo translation, it is alright as it is.